1. Field of the Invention
The present invention is directed to an improved, more efficient and economical process for the synthesis of urea from carbon dioxide and ammonia: 
This process involves reacting ammonia and carbon dioxide to form urea, followed by separation of unreacted ammonia and carbon dioxide by stripping with carbon dioxide at a pressure approximately equal to the urea synthesis pressure and by high and low pressure decomposition of residual ammonium carbamate. The process of the present invention efficiently uses off-gases from the high and low pressure decomposition and condensates of these gases and provides a more economical processes for synthesizing urea. For instance, improved conservation and recovery of heat provided by the present invention reduces process costs and improves the overall efficiency and economy of urea synthesis.
2. Description of the Related Art
Urea is synthesized by reacting carbon dioxide and ammonia at a high temperature and pressure. Conventionally, this reaction is performed inside of a urea synthesis zone or reactor at a suitable pressure and temperature for synthesis of urea and involves the formation and subsequent dehydration of ammonium carbamate to form urea: 
As shown above, the formation of ammonium carbamate and its subsequent dehydration into urea essentially occur simultaneously yielding urea. However, purity and yield of urea in the resulting reaction mixture (urea synthesis solution) can be improved by stripping using carbon dioxide and by high-pressure decomposition of ammonium carbonate.
Processes for obtaining urea from carbon dioxide and ammonia are known, see Japanese Patent Laid-Open No. 10-182587. Urea may be synthesized by the following process steps:
reacting ammonia and carbon dioxide at a urea synthesis pressure and a urea synthesis temperature to form a urea synthesis solution,
separating the unreacted ammonium carbamate from the urea synthesis solution as a gas mixture of ammonia, carbon dioxide and water by bringing the solution into contact with carbon dioxide at a pressure substantially equal to the urea synthesis pressure,
condensing the resulting gas mixture and recycling the condensate to the urea synthesis zone,
further treating the urea synthesis solution from which the unreacted ammonium carbamate is separated to obtain an aqueous urea solution, and
concentrating the aqueous urea solution.
In the above-described method, after stripping and condensation of the off-gases from the stripper, an uncondensed gas mixture containing inert gas, ammonia, carbon dioxide and water remains. This uncondensed gas mixture is washed in a scrubber using an absorption medium to absorb ammonia and carbon dioxide in the uncondensed gas mixture. After scrubbing substantially only the inert gas is discharged from the scrubber to the outside of the system.
Japanese Patent Laid-Open No. 61-109760, discloses that such an inert gas may be introduced into the high pressure decomposition step of the unreacted ammonium carbamate remaining in the urea synthesis solution after carbon dioxide stripping.
Other urea synthesis methods attempt to concentrate the aqueous urea solution formed by the above reactions using the condensation heat generated from condensation of the off-gases from the high pressure decomposition step, see e.g. Japanese Patent Publication No. 62-15070, Japanese Patent Laid-Open No. 63-112552, Japanese Patent Laid-Open No. 62-39559, Japanese Patent Laid-Open No. 60-166656, Japanese Patent Laid-Open No. 62-39560, Japanese Patent Laid-Open No. 63-126857, and EP A1 0329214).
However, the above-described methods do not describe the improved heat recovery, efficiencies and economies provided by the present invention, in which the off-gases from the high pressure decomposition of the unreacted ammonium carbamate in the urea synthesis solution are condensed in at least two steps, the condensate is recycled to the scrubber, and the uncondensed gases from the scrubber are routed as shown in FIGS. 1, 2 and 3.
One object of the present invention is to provide a process with improved heat economy for synthesizing urea that comprises stripping unreacted ammonium carbamate using carbon dioxide (e.g. raw material carbon dioxide) under a pressure equal to the urea synthesis pressure. In the present invention, the phrases, xe2x80x9ccondense a gas mixturexe2x80x9d or xe2x80x9ccondensing a gas mixturexe2x80x9d, may optionally encompass condensing and washing a gas mixture, condensing and absorbing a gas mixture, or both.
The above-described object of the present invention may be achieved by the following urea synthesis process:
(1) Reacting carbon dioxide, which may contain a slight amount of oxygen for corrosion prevention, with ammonia at pressure and temperature suitable for the synthesis of urea in a urea synthesis zone or reactor, thus forming a urea synthesis solution;
bringing the urea synthesis solution into contact with carbon dioxide at a pressure substantially equal to the urea synthesis pressure to separate a major or substantial part of the unreacted ammonium carbamate contained in the urea synthesis solution as a gas mixture of ammonia, carbon dioxide and water;
condensing the resulting gas mixture and recycling the condensate into the urea synthesis zone or reactor;
subjecting the urea synthesis solution having a major or substantial part of the unreacted ammonium carbamate removed to a high pressure decomposition, preferably at about 1 to 4 Mpa, thereby separating unreacted ammonium carbamate remaining in the urea synthesis solution as a gas mixture of ammonia, carbon dioxide and water;
subjecting the resulting urea synthesis solution containing the remaining unreacted ammonium carbamate to low pressure decomposition, preferably at about 1 to 0.5 MPa, in at least one stage, thereby separating the substantially all of the remaining unreacted ammonium carbamate as a gas mixture of ammonia, carbon dioxide and water to obtain an aqueous urea solution;
cooling and condensing the low pressure gas mixture separated in the low pressure decomposition to obtain a low pressure condensate;
condensing the off-gases from the high-pressure decomposition of ammonium carbamate by:
contacting them with a condensate of the off-gases from the high and low pressure decompositions of ammonium carbamate, and
by indirectly exchanging heat with the aqueous urea solution to condense the high pressure gas mixture to obtain condensate of the gases from the high-pressure decomposition;
utilizing the condensation heat generated at that time for at least a part of the heat source for concentrating the aqueous urea solution; and
introducing the high pressure condensate (see e.g. line 6 in FIGS. 1, 2 and 3) into the condensation step for the gas mixture obtained from stripping the urea synthesis solution with carbon dioxide at a pressure substantially equal to the urea synthesis pressure (see e.g. element xe2x80x9cBxe2x80x9d in FIGS. 1, 2, and 3);
wherein the high pressure gas mixture resulting from the high pressure decomposition of unreacted ammonium carbamate is condensed by indirect heat exchange with the aqueous urea solution (e.g. condensing element K in FIGS. 1, 2 and 3), followed by at least two condensations and a washing (e.g. as respectively shown in P, Q and R in FIGS. 1, 2 and 3).
The condensation of the high-pressure gas mixture or off-gases resulting from the high-pressure decomposition may be carried out by:
condensing the high-pressure gas mixture from the high-pressure decomposition step (e.g. line 20 in FIGS. 1, 2 and 3) by mixture with condensates of gases obtained from the high and low pressure decomposition of ammonium carbamate and by indirect heat-exchange with aqueous urea solution (see e.g. xe2x80x9cKxe2x80x9d in FIGS. 1, 2 and 3);
condensing remaining high-pressure gas mixture in a first condensation zone or first condenser (e.g. xe2x80x9cPxe2x80x9d in FIGS. 1, 2 and 3) and recycling the liquid condensate to a scrubber (e.g. scrubber xe2x80x9cFxe2x80x9d in FIGS. 1, 2 and 3).
condensing remaining high-pressure gas mixture from the prior condensation step in a second condensation zone or second condenser (e.g. xe2x80x9cQxe2x80x9d in FIGS. 1, 2 and 3),
bringing any remaining uncondensed ammonia and carbon dioxide into contact with the pressurized, low-pressure condensate from low-pressure decomposition of unreacted ammonium carbamate (e.g. from col. H in FIGS. 1, 2 and 3) in the step of washing (e.g. xe2x80x9cRxe2x80x9d in FIGS. 1, 2 and 3) and optionally discharging inert gas from the washing step.
Other objects of the present invention include:
(2) The process as described above in section (1), wherein the uncondensed gas mixture containing inert gas, ammonia, carbon oxide, and water obtained from stripping the urea synthesis solution (e.g. from stripper xe2x80x9cBxe2x80x9d in FIGS. 1, 2 and 3) and from the scrubber (e.g. scrubber xe2x80x9cBxe2x80x9d in FIGS. 1, 2 and 3) is introduced into the high pressure decomposition (e.g. xe2x80x9cGxe2x80x9d in FIG. 1) to decompose the unreacted ammonium carbamate.
(3) The process as described in above in section (1), wherein the uncondensed gas mixture containing inert gas, ammonia, carbon oxide, and water obtained from stripping the urea synthesis solution (e.g. from stripper xe2x80x9cBxe2x80x9d in FIGS. 1, 2 and 3) and from the scrubber (e.g. scrubber xe2x80x9cBxe2x80x9d in FIGS. 1, 2 and 3) is introduced into the step of condensing by indirect heat exchange (e.g. xe2x80x9cLxe2x80x9d in FIG. 2) to condense at least a part of ammonia, carbon dioxide and water in the gas mixture.
(4) The process as described in above in section (1), wherein the uncondensed gas mixture containing inert gas, ammonia, carbon oxide, and water obtained from stripping the urea synthesis solution (e.g. from stripper xe2x80x9cBxe2x80x9d in FIGS. 1, 2 and 3) and from the scrubber (e.g. scrubber xe2x80x9cBxe2x80x9d in FIGS. 1, 2 and 3) is introduced into the step of first cooling and condensing (e.g. xe2x80x9cPxe2x80x9d in FIG. 3) to condense at least a part of ammonia, carbon dioxide and water in the gas mixture.
In the present invention, gas from a high pressure decomposition column is condensed in at least two steps (or in three steps including the condensation occurring in the aqueous urea solution heating apparatus). Moreover, gas from a scrubber may also be condensed together with the gas from the high-pressure decomposition. Thus, the temperature in the condensing part of the aqueous urea solution heating apparatus can be increased and heat recovery can efficiently be carried out. Owing to the presence of oxygen from the scrubber, apparatus corrosion can also be avoided.
Further, the present invention allows the use of condensates having a pressure equal to the pressure of the high pressure decomposition column as an absorption medium for gases from the high pressure decomposition column without requiring a further increase of pressure.